1. Field of the Invention
The present invention relates generally to an apparatus and a method for acquiring frame synchronization in a mobile communication system, and in particular, to an apparatus and a method for acquiring frame synchronization in an asynchronous mobile communication system.
To be specific, the present invention relates to an apparatus and a method for synchronizing a 20 msec frame of a broadcasting channel (BCH) in a primary common control physical channel (PCCPCH), which is a downlink common physical channel, by a user equipment (UE) in an asynchronous mobile communication system.
2. Description of the Related Art
The following is a description of a next generation asynchronous mobile telecommunication system made with reference to a universal mobile telecommunication system/3rd generation partnership project (UMTS/3GPP).
To date, no specification of 3GPP has described acquiring frame synchronization of BCH, and no prior art specifically defines the same because it is a matter of embodiment. The following is a description of a process of synchronizing a 10 msec frame through initial cell detection by the UE in the asynchronous mobile communication system.
FIG. 1 is a diagram illustrating the structure of a primary synchronization channel, a secondary synchronization channel, and a common pilot channel (CPICH). The Synchronization Channel (SCH) is a downlink signal used for cell search (first cell searching step). The SCH consists of two sub channels, the Primary and Secondary SCH. The 10 ms radio frames of the Primary and Secondary SCH are divided into 15 slots, each of length 2560 chips. The Primary SCH consists of a modulated code of length 256 chips, the Primary Synchronization Code (PSC) denoted cp in FIG. 1, transmitted once every slot. The PSC is the same for every cell in the system.
The Secondary SCH consists of repeatedly transmitting a length 15 sequence of modulated codes of length 256 chips, the Secondary Synchronization Codes (SSC), transmitted in parallel with the Primary SCH. The SSC is denoted cSi, k in FIG. 1, where i=0, 1, . . . , 63 is the number of the scrambling code group, and k=1, 2, . . . , 15 is the slot number. Each SSC is chosen from a set of 16 different codes of length 256. This sequence on the Secondary SCH indicates which of the code groups the cell's downlink scrambling code belongs to. The UE, which has been synchronized with the time slot in the above first cell searching step, detects the code group data and a 10 msec frame synchronization through the secondary synchronization channels. Here, the code group data of the base station is the data determining a code group where the base station belongs, and is designated in accordance with a “comma free code” (second cell searching step).
FIG. 2 is a diagram illustrating a frame timing relationship among downlink physical channels in a mobile telecommunication system. The Primary CCPCH is a fixed rate (30 kbps, SF=256) downlink physical channels used to carry the BCH transport channel.
As described above, the UE synchronizes a 10 msec frame (or a system timing) through an initial cell search consisting of two steps. Thereafter, the UE needs to promptly decode a broadcasting channel (BCH) for prompt launch of the service. However, a 10 msec frame only has yet been synchronized at present, and no frame boundary has yet been known with respect to the 20 msec frame to decode the 20 msec BCH frame. Under these circumstances, if a radio frame of PCCPCH of a 10 msec unit is randomly decoded for 20 msec (=TTI of BCH) and uploaded to an upper layer, there is no guarantee that the 20 msec interval corresponds to the 20 msec boundary of the BCH. Therefore, if the first frame boundary is not matched, incorrect decoding data is consecutively uploaded on the upper layer. In other words, in order to promptly decode the BCH frame, it is necessary to upload the BCH frame data, which has been decoded without any errors, on the upper layer by acquiring a frame boundary of 20 msec BCH on a layer 1.